The present invention relates to a printer and, more particularly, to a stencil printer of the type wrapping a master around a print drum and pressing a sheet against the master with the print drum or a press drum.
Various kinds of sheet feeding devices have heretofore been proposed for use in a stencil printer of the type described. One of them uses a sector gear for feeding a sheet from the top of a stack loaded on a sheet tray to a gap between a print drum and a press roller. Another conventional sheet feeding device includes a press drum having substantially the same outside diameter as the print drum i n place of the press roller. The press drum is rotatable at substantially the same peripheral speed as the print drum, but in the opposite direction to the print drum, while clamping the leading edge of a sheet thereon. The press drum therefore forcibly separates the leading edge of the sheet, or printing, from the print drum. With this kind of press drum, it is possible to obviate an occurrence that the leading edge of the sheet is not peeled off by a peeler, causing the sheet to roll up and jam. It is also possible to reduce noise and to enhance accurate positioning of an image on the sheet in the direction of sheet transport (registration accuracy).
The conventional sector gear scheme and press drum scheme each has some problems left unsolved, as follows. The sector gear scheme cannot maintain the positional accuracy or registration accuracy in the direction of sheet transport, and needs a main motor whose output power is great enough to withstand a heavy load, as will be discussed more specifically later. The press drum scheme is not practicable without resorting to a drive transmission system between the print drum and the press drum and involving a top-down adjusting mechanism made up of a number of parts, as taught in, e.g., Japanese Patent Laid-Open Publication No. 9-216448. This, coupled with the fact that the above drive transmission system has a substantial length to a main motor, disturbs the timing for the sheet to be driven from a registration roller toward a sheet clamper provided on the press drum, lessening the effect of the press drum scheme. The disturbance is ascribable to backlash particular to a gear train included in the top-bottom adjusting mechanism and the slackening or stretching of a timing belt. As a result, the sheet is caused to roll up, as stated earlier.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-330225, 8-332769, 9-1914, 5-305707 and 6-247586, Japanese Utility Model Publication No. 6-20620, and U.S. Pat. Nos. 5,415,387 and 4,911,069.